The invention relates to a fuel fractionation device for separating a low-boiling fuel fraction from a liquid fuel of an internal-combustion engine in a container, in which a low pressure is generated for evaporating the low boiling fuel fraction.
The fuels which are currently available for the operation of internal-combustion engines, for example for motor vehicles such as trucks, passenger cars, buses, represent a compromise between, firstly, restrictions on the part of fuel producers, e.g. on account of different crude oil qualities, production methods, costs and energy expenses, and, secondly, in some cases contradictory demands imposed by the internal-combustion engines, such as for example a reliable cold start even at extremely low temperatures, low exhaust and evaporation emissions, low fuel consumption, good antiknock properties even for turbocharged engines, prevention of deposits, avoidance of corrosion, low sulfur content, quiet engine running and high reliability. To enable existing fuels to be better adapted to the requirements of the internal-combustion engines, fuel fractionation devices of the type described above are used.
DE 197 13 841 C1 has disclosed a fuel fractionation device of this type in which in a fuel vessel, which may be formed either by a fuel tank of a motor vehicle or by an intermediate vessel, a reduced pressure is generated as a result of gas being sucked out. In the known device, this reduced pressure is selected in such a way that the components of the low-boiling fuel fraction, which are to be separated from the liquid fuel contained in the fuel vessel, evaporate out of this fuel. In this case, the extraction of gas or the generation of the reduced pressure is achieved by means of a gas pump. Moreover, with this extraction of gas also the low-boiling fuel fraction in vapor form which is formed in the vessel is withdrawn from the vessel. On the delivery side of the gas pump, the evaporated components are condensed, in order to be able to provide the low-boiling fuel fraction, which has been separated, to the internal-combustion engine in a liquid state. To improve the performance of the fuel fractionation device, in particular the fractionation rate, the suction side of the gas pump can be heated and/or the pressure side of the pump can be cooled.
The present invention deals with the problem of improving the performance of a fuel fractionation device of the type described in the introduction. It is a particular object to increase the fractionation rate.
In a fuel fractionation device for separating a low-boiling fuel fraction from a liquid fuel of an internal combustion engine, including a vessel which contains the liquid fuel, and in which a reduced pressure is generated by withdrawing the low-boiling fuel fraction in vapor form from the vessel and making it available to the internal combustion engine, the efficiency of the fractionation is improved by utilizing a carrier-gas which is introduced into the liquid fuel, and which is withdrawn from the vessel together with the fuel fraction vapor.
The carrier gas rises through the fuel in the form of bubbles and, in the process, is preferentially enriched with the low-boiling fuel components. The mixture collects above the fuel level in the vessel, from where the carrier gas/fuel fraction mixture formed in this way can be moved out of the vessel. Unlike in the prior art, the reduced pressure, which is in this case generated in the vessel by the gas being sucked out, only has to ensure that the gaseous carrier gas/fuel fraction mixture formed in the vessel is sucked out of the vessel. The pressure reduction required to achieve this is considerably lower than the pressure reduction required to evaporate the components of the low-boiling fuel fraction out of the liquid fuel. The pressure source, which is used to generate the reduced pressure and also the vessel construction, can in this way be simplified.
Preferably, however, the reduced pressure, which is generated in the vessel as a result of the gas being sucked out, is large enough to suck in the carrier gas, so that there is no need for an additional pump. By way of example, the suction requirements are low enough so that they can be satisfied by the vacuum generated in the air intake of the internal-combustion engine. In the process, at least some of the air required for combustion in the internal-combustion engine is sucked in through the vessel. In this case, the carrier gas used is ambient air.
According to a particularly advantageous embodiment using air as carrier gas, the air/fuel fraction mixture extracted from the vessel can be fed to the internal-combustion engine directly as a combustible air/fuel mixture, since the air, which is enriched with the low-boiling fuel fraction, forms an ignitable gas mixture.
Alternatively, the gas may also be sucked out by means of a special gas pump, so that it is also possible to compress the carrier gas/fuel fraction mixture on the delivery side of the gas pump.
Instead of an open circuit, in which the air used as carrier gas is fed to the internal combustion engine for combustion together with the fuel fraction which has been separated off in the form of an air/fuel fraction mixture, in which case the carrier gas is consumed, it is also possible to form a closed circuit, in which, after the carrier gas/fuel fraction mixture has been sucked out, the fuel fraction is separated from the carrier gas and is used again. In this case, the fuel fraction, which is separated out of the mixture is held ready in a special reservoir, preferably collected in the liquid state, and is fed into the internal-combustion engine when required. The carrier gas may either be introduced back into the fuel in the vessel or, in particular if air is used as carrier gas, can be discharged the environment.
In accordance with a particularly advantageous embodiment of the fuel fractionation device according to the invention, the vessel into which the carrier gas is introduced and from which the carrier gas/fuel fraction mixture is sucked out may be formed by a fuel tank in which the fuel for the internal-combustion engine is stored. In the case of a vehicle, this is the vehicle""s fuel tank for supplying fuel to the internal-combustion engine may be employed for that purpose.
The invention will become more readily apparent from the following description of a preferred embodiment on the basis of the accompanying drawings.